Apparatus for mixing a multi-component encapsulant and injecting it through a heated nozzle onto a part to be encapsulated

ABSTRACT

The present invention is an apparatus and method for mixing a plurality of materials in-situ to form an admixture and heating a localized portion of admixture upon dispense. The heat thereby primarily effects the speed of chemical reaction and curing of only the exiting reactants with little effect upon the admixture not dispensed. The apparatus and process are especially useful for encapsulating a plurality of workpieces at a high throughput. Advantageous configurations are described such as wherein the workpieces and/or the dispensing mechanism may be mounted on a carrier and fed by an assembly line. Alternative admixture chemistries are described.

This is a continuation of application Ser. No. 08/615,913, filed Mar.14, 1996.

This application claims priority from Provisional Application Serial No.60/007,837 which was filed Dec. 1, 1995.

FIELD OF THE INVENTION

The present invention is directed to an apparatus and method for in-situmixing, heating and dispensing of materials. More particularly, it isalso directed to the application of a heated mixture to devices on anassembly line.

BACKGROUND OF THE INVENTION

It is a constant endeavor to find improved ways to apply resins toencapsulate components, chips and/or modules. Presently chip or glob topencapsulation is generally accomplished with premixed materials.Pitfalls associated with this method range from improper resin storage,difficulty of flow control, and improper tube or syringe properties forapplication requirements. In order to extend the mixture's shelf life,it has to be frozen and subsequently thawed for use. It is absolutelyessential to maintain the mixture at its required temperature. Failureto maintain proper storage temperature causes accelerated aging. Thisoften results in increased material viscosity requiring ejection at ahigh pressure.

Most encapsulation methods presently employed require long cure cycles.A long cure cycle is particularly costly and inappropriate for use in anautomated assembly line for a high volume throughput. This is evident byconsidering a typical assembly line situation. An assembly line with aline speed of 12 feet/min, and a cure time at raised temperature of 10minutes, requires an oven at least 120 feet long. Currently, propercuring of a one pot, premixed encapsulation material takes about twohours at a raised temperature. To partially overcome this problem fordevices mounted on a continuous tape-like strip, the parts are rolledafter the encapsulating material, the encapsulant, has gelled. This isto prevent the parts from flowing, peeling or sticking to each other. Inaddition, the parts have to be cut and placed in a cassette for postcuring.

Successful use of a one pot reactant material mixture in a large volumeencapsulation application is difficult. In any one pot mixture, areaction is progressing at all times. Increasing the temperature of thereacting system accelerates the reaction, while cooling slows thereaction. The actual change in the reaction rate is dependent on theviscosity. Intrinsically it depends upon the activation energy for thereaction. The reaction rate approximate doubles with every 10°Centigrade increase in temperature. The use of very high temperatures toforce a reaction to proceed extremely quickly is limited by the thermalstability of the reactants and products of the reaction. For aliphaticcontaining epoxy resins, this temperature is limited to about 170° C.Aromatic epoxy resins can sustain substantially higher temperatures, butthe reactivity of these species is substantially slower than foraliphatic systems and some reaction chemistries are not suitable foraromatic based resins.

One solution is to include a very efficient and fast catalyst system inthe filled epoxy resin used for chip encapsulation. Shelf life becomes avery significant problem if the catalyst is very fast. Methods anddevices to mix two liquids with and without the simultaneous applicationof thermal energy exist. Unfortunately, none can be successfully used inan application requiring very fast reaction. The problem of uniformheating throughout the mixing chamber was addressed by Griffith, U.S.Pat. No. 4,678,881, by using a heated paddle. The present inventionpresents a solution in which the catalyst is mixed with the resin onlyimmediately prior to its being dispensed to encapsulate a device.

Inoue, U.S. Pat. No. 4,834,545, addresses the problem of mixing anddispensing an admixture of two materials by utilizing a conical mixingchamber and axial motion of the mixing spindle to eject the resin. Thereis no provision for heating. If the entire chamber was heated, residualresin in the mixing chamber following ejection would cure and result inbuildup and eventually clogging and poor dispense column control.

Continuous reactors, such as that presented by Wilt, U.S. Pat. No.4,438,074, typically have very long residence times with temperaturecontrol. There is no means of dispensing controlled amounts of theproduct and certainly no means for heating the mixture after dispense.These systems are typically used in mixtures which include a solvent.The resultant lower viscosity mixture does not require large shearingforces to be dispensed from the mixer. Materials requiring relativelylarge shearing forces preclude many means for dispensing the resin. Thepresent invention is applicable even to filled materials which generallymay require large shearing forces to be dispensed.

Haeuser, U.S. Pat. No. 4,741,623, shows a means for pumping a liquidinto a mixing chamber, but gives no indication as to how the reactantsare mixed or dispensed from the mixing chamber or subsequently heated.

Japanese patent, JP7833275, describes the mixing of two reactants andinjection into a mold. This patent does not concern itself withcontrolled dispense or heating the reactants upon exit. The presentinvention used in conjunction with the device described in this patentwould result in the resin curing within the mixing chamber and cloggingit.

It is an object of the present invention to provide a means that cansatisfactorily deliver a controlled volume of fast curing resin in avery precise and consistent manner around a device. This would eliminatethe need for very long and expensive tunnel ovens in assembly lineapplications. It is another object to use epoxy resins that are highlyfilled to attempt to better match the coefficients of thermal expansion(CTE) of the resin and the chip. A very heavy loading of greater than 50volume percent decreases the mixture's CTE to about 30 ppm as comparedto about 60-80 ppm for the neat resin. The chip's CTE is about 2 ppm.The CTE of the lead frame on which the chip is mounted is about 18-25ppm. By reducing the CTE mismatch, residual stress is reduced in theresulting structure.

It is another object of the present invention to reduce the resin'sviscosity by heating it prior to its being applied to the chip. Thisallows the resin to settle around the chip and to effectivelyencapsulate the chip without leaving substantial voids or bubbles. Italso reduces the time necessary to reheat the resin on the chip therebyreducing the necessary curing time. It enables the reaction to startprior to deposition on the chip. This is advantageous since viscosityincreases very slowly in the initial stages of a reaction. The rate ofincrease of the viscosity increases with the extent of reaction untilthe gel point is reached. At this point the viscosity is essentiallyinfinite. Prior heating also helps the small amount of resin to dropproperly from the tip of the applicator. Proper resin dropping isrelated to surface tension and viscosity which decrease withtemperature.

If the mixer or the stirrer in the mixer is heated in entirety, thechemistry starts prematurely. Premature heating would cause theformation of surface coatings even if the resin is flushed out of themixer often. The coatings gel and become permanent within the mixer.Ultimately, the thickness of this coating becomes sufficient tocontaminate the mixer. This would require that the mixer be disassembledand cleaned or replaced.

The present invention overcomes these problems and provides a method andapparatus which may be employed in extremely high volume throughputapplications. The encapsulating material is mixed in-situ which leads toa shortened cure time in the order of several minutes or less, forexample five minutes. This process also provides improved and increasedprocessing capability and longer material pot life.

SUMMARY OF THE INVENTION

The present invention is directed to in-situ mixing and dispensing ofmaterials. It is specifically directed to the local application of heatto the dispensing portion of a reactant admixture simultaneous with thedispensing so as to speed up the curing of the dispensed materialswithout effecting the materials not dispensed. It is still morespecifically directed to the application of the heated dispensedadmixture to components on an assembly line. It is desirable that thecomponents being encapsulated are preheated.

A broad aspect of the present invention is an apparatus having a mixingchamber which includes means for receiving and mixing materials to forma combination substance. It also includes means for dispensing thecombination substance from an output port onto a workpiece, and meansfor raising the temperature of the dispensed combination substance incorrespondence with the means for dispensing. Generally, the combinationsubstance includes a reactive curable mixture of polymers. It isdesirable for the dispensing means to include means for controlling theamount of combination substance dispensed, means for controlling theposition, time and duration for the dispensing to occur, means todispense a drop, or an elongated or otherwise shaped quantity of thecombination substance, and means for positioning each workpiece at aposition to receive the combination substance being dispensed. Desirablemeans for raising the temperature includes RF induction heating andresistive heating.

Another aspect of the present invention includes an assembly line meansfor carrying and moving each workpiece to a position to receive thecombination substance and/or for moving the dispensing apparatus to theposition of each device receiving the combination substance.

Still another aspect of the present invention is an apparatus forcontrollably dispensing on a workpiece a curable material which is atleast partially cured. The apparatus includes a container for thematerial, means for controlling an amount of material dispensed througha dispensing port and means for at least partially curing the materialas the material is dispensed onto the workpiece.

Another broad aspect of the present invention is a method forcontrollably dispensing on a workpiece a curable material which is atleast partially cured. The method includes the step of controllablyejecting the curable material towards the workpiece while controllablycuring the curable material so that the workpiece receives the curablematerial at a predetermined degree of cure.

Another method of the present invention is a process for dispensing acombination substance formed from a plurality of materials. The methodincludes the steps of mixing and dispensing the substance and raisingthe temperature of the substance in correspondence with its beingdispensed. It is desirable that the method also include the steps ofproviding workpieces and applying the substance dispensed onto each ofthe workpieces especially when the substance is for encapsulating eachworkpiece.

Another aspect of the present invention is a method which includes stepsof sequentially feeding the workpieces to receive the substance andcontrolling the type, amount, position, time and duration of dispense.This is desirably performed using an assembly line means and having theworkpieces mounted on a reel.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects, features, and advantages of the presentinvention will become apparent upon further consideration of thefollowing detailed description of the invention when read in conjunctionwith the drawing figures, in which:

FIG. 1 shows a schematic diagram of an apparatus of the presentinvention.

FIG. 2 shows a schematic diagram of the apparatus of FIG. 1 with moredetail.

FIG. 3 shows an enlargement of the ejection end of the apparatus of FIG.2 with an elongated ejected curable material being disposed on a workpiece.

FIG. 4 shows a top view of a use resulting from the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is an apparatus and process for encapsulating aworkpiece with a material formed by mixing a resin and a catalystin-situ. The components are fed into a mixing chamber in controlledvolumes wherein they are intimately mixed. The admixture is fed into aconnecting chamber which contains a delivery device to accurately andconsistently deliver a controlled volume of admixture. According to thepresent invention only a small portion of the admixture is heatedimmediately prior to, upon exiting, or during delivery from the deliverydevice. It is advantageous to also preheat the workpiece beingencapsulated to reduce moisture absorption and enhance adhesion.Minimally, the resin should cure beyond the gel point, but ideally, itshould cure completely. At the gel point the resin has essentiallyinfinite viscosity and cannot flow or creep. It is desirablyaccomplished within the time it takes to accumulate the resultingencapsulated workpieces by rolling or placing them in a cassette. Thisresults in the encapsulation of the workpiece with a controlled volumeof resin in a reduced cure time, thereby, making the process applicableto extremely high throughput assembly line requirements. Although theprocess is described for mixing two components, it is applicable to themixing of three or more components.

The process and apparatus are schematically shown in FIG. 1, in whichtwo reactants are stored in and delivered from reservoirs 22 and 24 to amixing device 25. The mixing device 25 mixes the reactants fromreservoirs 22 and 24 and dispenses the admixture 26 of the reactants. Adrop 28 of the admixture 26 is heated upon exit from dispense port 27and dropped onto a moving conveyor 30 carrying the workpieces 31-34 tobe encapsulated. It is preferable to have the workpieces mounted on atape reel for feeding 35 and a tape reel for uptake 36. Workpiece 31 isshown to be encapsulated after having already passed the dispense port27. Workpiece 32 is being encapsulated. Workpieces 33-34 are to beencapsulated. The process is controlled by a controller 20 whichoverseas the operation and controls the time of dispensing drop 28 tocorrespond with the passing of a workpiece to be encapsulated under thedispense port 27. A heating means 29 is provided for raising thetemperature of drop 28. Due to the fast chemical reaction of the heateddrop 28, the time for curing the encapsulated workpiece 31 is veryshort. It is desirable that the dispensing apparatus and heatingmechanism work cooperatively in such a way as to isolate the heatgenerated from the major portion of substance not being ejected.

A more detailed embodiment of a method and apparatus practicing thepresent invention is illustrated in FIG. 2. FIG. 2 shows a mixing systemstructure 100 in which the input streams of two components 102 and 104enter a mixing chamber 106 through flow controllers 108 and 110. Theflow can also be controlled using a linear pump at the source of each ofthe components to be mixed. This would eliminate the need for flowcontrollers 108 and 110. It is desirable that the pressure of thecomponents 102 and 104 be the same as the pressure in the mixing chamber106. The components 102 and 104 are intimately mixed using a stirrer 112which has a hollow shaft 113 and is connected to a high torque mixingmotor 114. Desirably, the mixing motor 114 moves at a substantiallyconstant, predefined speed in a particular direction. The paddles 116 onthe mixing system are designed to exert a pressure in the downwarddirection towards the dispense head 117. The dispense head 117 includesa delivery stage 118 and a heating means 123. The delivery stage 118 isadjacent and connected to the mixing chamber 106. The delivery stage 118contains a screw extruder 120 which when turned in a specific directionforces the contents 119 of the delivery stage 118 through an orifice122. The orifice 122 is preferably such as to form a desired shape,width and elongation of the dispensing substance. The screw extruder 120is controlled by a precision motor 124 through the hollow center shaftof the stirrer 112. It is preferably employed to control the amount ofthe substance being dispensed and/or the duration of the dispense. Insome cases the amount may be as little as 0.1 to 1 ml. When turned inthe opposite direction, the screw extruder 120 causes material remainingin the delivery chamber to be sucked back into the mixing chamber. Theextruder motor 124 is preferably a stepping motor. When a droplet orother shape of material is required, the structure controller 120 issuesa signal to turn the screw extruder in proportion to the volume of thedroplet required. The droplet 121 is heated by a heating means 123 whichtends to localize the region of resin temperature increase to thedroplet. Depending upon the heating method employed, the droplet 121 isheated just prior to its being dispensed, coincident with its beingdispensed and/or immediately after its being dispensed from the orifice122. It is desirable that the dispensing apparatus and heating means 123work cooperatively in such a way as to isolate the heat generated awayfrom the major portion of substance not being ejected.

One means for accomplishing this localized heating is the use of RFinduction heating. In this technique, the heating element 126 is acoiled wire through which RF radiation is passed. Use of a tip 125 tosupport the heating element 126 is optional. This creates an alternatingelectric field within the region of the coil and slightly beyond, alongthe center axis of the coil. RF radiation couples energy to the resindrop 122 passing through it by a variety of mechanisms, includingdipolar relaxation, ionic conductivity or resistive heating ofconductive elements. The preferred admixture material for thisapplication contains carbon black as a filler and coloring agent. Carbonblack is partially conductive, resulting in rapid heating in an RFfield. Desirably, the RF radiation is controlled by the controller 120and is only turned on when resin is passing through the orifice 122.

Another means for heating the resin on exit is to use resistive heatersas the heating element 126 along a tip 125. In this case, a screwextruder 120 is turned to force a desired amount of resin, drop 121,through orifice 122 and out the heated region. Following dispense of thedrop 121, the extruder motor 124 is set to perform a reverse turn toremove any excess material in the heated region in a very short amountof time. This is to prevent premature chemical reaction of the excessadmixture. When the next drop of admixture is due, the extruder motor124 drives to account for the material pulled back from the previousdrop in addition to the volume required for the next drop. Other methodsof providing thermal energy to the dispensing substance may be used.

In the means described the mixing structure is stationary and theworkpieces to be encapsulated are moved along an assembly line. Theinventive process is similarly applicable to a moving mixing structure.Thus, in another embodiment of the present invention, rather than theworkpieces to be encapsulated moving on a conveyor, the mixing structurecan be placed above an X-Y-Z table on which the workpieces to beencapsulated are placed, or the mixing structure is itself movable.

FIG. 3 shows the apparatus of FIG. 2 used to dispense a continuousamount of admixture rather than a droplet as shown in FIG. 2. In FIG. 3,the mixing structure 200 causes a continuous line 204 of heated adhesiveto be dispensed by the extruder 202 onto a substrate 206. The dispensingport 222 may be such that the width of the line of dispensed adhesive isnarrow or extremely broad as required by the particular utilization. Aflexible body 210 can be attached to the substrate 206 with the use ofminimal pressure applied by a roller 212. This forms a bond-line 208 toadhere the flexible body 210 to the substrate 206. The resin cures inplace to form a strong high temperature adhesive joint without requiringthat the entire assembly be heated to cure the adhesive. A top view of aflexible body 210 adhered to a substrate 206 by a bond-line 208 is shownin FIG. 4.

Many alternative configurations of the present invention can be usedaccording to the needs of the particular utilization. For example, itcan be used to vary the volume of material dispensed from drop to dropby controlling the screw extruder. It can also be used with theadmixture caused to run in a continuous mode in which the volume perunit time is changing in a controlled manner. Varying the bond thicknessalong a bond line could result in adherence with near optimal strength.Multiple dispense heads emanating from a common mixing chamber could beemployed simultaneously. A single moving head can be used in a conveyorsystem. The head moves on an axis substantially perpendicular to thedirection of motion of the conveyer to deliver the admixture to a numberof distinct workpieces on the conveyor. Single or multiple dispenseheads can be used to dispense admixture over a surface of a firstsubstrate to which a second substrate is being adhered. The resultingsingle or multiple bond-lines being predetermined and controlled inaccordance with a preprogrammed manner. It is noted that thecross-sectional shape of the material delivered does not have to becircular. It can be set to depend upon the shape of the final orifice.This is especially so, when the admixture is being dispensed in acontinuous mode. The mixing chamber and the dispense head need not bearranged coaxially. The present invention will work just as well if themixing chamber is separated from the dispense head as long as thestirrer is designed to direct the admixture material towards thedelivery stage and wherein a means is employed to speed the chemicalreaction of only a small portion of substance at the point of ejection.

A range of resin chemistries can be used which can result in anadmixture which cures very rapidly. A material of choice is an aliphaticepoxy resin with a catalyst. The actual admixture cure time can varywidely depending upon the catalyst type and its concentration as shownin the examples below. For purposes of the examples, the curetemperature was standardized at 100° C. It is very well known in the artthat increasing temperature will reduce the cure time.

It is generally desirable for the resin to be filled with a materialwhich performs a number of tasks. These tasks include: eliminating lightfrom reaching the encapsulated workpiece; protecting the encapsulatedworkpiece from mechanical damage; preventing absorption of excessivemoisture which could corrode the package metallurgy; capturing heatenergy; etc. To accomplish these tasks, the resin is usually filled withan opaque material such as carbon black which also provides someantistatic benefits. It is noted, that the viscosity of the admixtureincreases substantially when carbon black or another filler is added.Difficulty in mixing the resulting very viscous components can beovercome by the correct choice of mixing blades as in commonly known tothose skilled in the art of chemical engineering.

A group of polymeric materials desirably employed in accordance with thepresent invention are cycloaliphatic epoxides.

Cycloaliphatic epoxides are known and are obtained by oxidation ofcyclic olefins. Cycloaliphatic epoxy materials are generally relativelyslow reacting particularly as compared to the more conventional glycidylether type of epoxides. Examples of suitable cycloaliphatic epoxides aresuggested in U.S. Pat. Nos. 3,027,357; 2,890,194; 2,890,197; and4,294,746, disclosures of which are incorporated herein by reference.Some specific examples of suitable cycloaliphatic epoxides are:3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexane carboxylate, availablefrom Union Carbide under the trade designation ERL-4221; bis(3,4-epoxy-6-methylcyclohexylmethyl) adipate; bis(3,4-epoxycyclohexylmethyl) adipate; bis (2,3-epoxycyclopentyl) ether;vinyl cyclohexane diepoxide, available from Union Carbide under thetrade designation ERL-4206;2-(3,4-epoxycyclohexyl)-5,5-spiro(2,3-epoxycyclohexane)-m-dioxane;2-(3,4-epoxycyclohexyl-5,5-spiro-3,4-epoxy) cyclohexane-m-dioxane,available from Union Carbide under the trade designation ERL-4234; andbis (3,4-epoxycyclohexyl) adipate, available from Union Carbide underthe trade designation ERL-4299.

A discussion of various cycloaliphatic epoxides may be found in thepublication entitled "Cycloaliphatic Epoxide Systems", Union Carbide,1970, disclosure of which is incorporated herein by reference.

Mixtures of cycloaliphatic epoxides can be employed when desired. Apreferred cycloaliphatic epoxide employed pursuant to the presentinvention is 3,4-epoxycyclohexylmethyl 3,4-epoxycyclohexanecarboxylate(systematic name: 7-oxabicyclo 4.1.0!heptane-3-carboxylic acid7-oxabicyclo 4.1.0!hept-3-ylmethyl ester)

The compositions of the present invention include an anhydride of anorganic carboxylic acid as a hardening agent. The hardening agent ispreferably in liquid form. If a solid anhydride hardening agent isemployed, such should be melted when added to the composition. Examplesof anhydrides are methyltetrahydrophthalic anhydride; hexahydrophthalicanhydride; maleic anhydride, trimellitic anhydride; pyromelliticdianhydride, tetrahydrophthalic anhydride; phthalic anhydride;norbornenedicarboxylic anhydride; nadic methyl anhydride; andmethylcyclohexane-1,2-dicarboxylic anhydride.

Additional anhydrides can be found, for instance, in H. Lee and K.Neville, Handbook of Epoxy Resin, McGraw Hill, 1967, Chapter 12,disclosure of which is incorporated herein by reference.

An anhydride curing agent is generally employed in amounts constitutingon an equivalent basis, about 20% to about 120% of the cycloaliphaticepoxide employed and desirably about 75% to about 100% of the epoxideequivalents.

In addition to the cycloaliphatic epoxide and hardener, a reactivemodifier can also be used. The purpose of the reactive modifier is toimpart desirable mechanical properties to the cured composition such asflexibility and thermal shock resistance. Examples of modifiers whichcan be used are fatty acids, fatty acid anhydrides such as polyazelaicpolyanhydride and dodecenylsuccinic anhydride, long chain alcohols,polyetherdiols such as polyethylene glycol and polypropylene glycol, andother materials having hydroxyl, carboxyl, epoxy, and/or carboxylicanhydride functionality.

In order to provide a composition having reaction times fast enough tobe compatible in a reaction injection molding process, it is usuallynecessary to employ as reaction or hardener promoter an alkylene oxidesubstituted adduct of an imidazole. The alkylene oxide employed can beethylene oxide, propylene oxide, or butylene oxide and is preferablypropylene oxide. The imidazole employed is preferably imidazole.However, if desired, substituted imidazoles can be used, including alkylsubstituted such as C₁ -C₁₂ alkyl groups. When a substituted imidazoleis employed as the imidazole it is usually substituted at the carbonatoms of the imidazole ring and more usually at positions 4 and/or 5thereof. Mixtures of imidazoles as well as mixtures of alkylene oxidescan be used. The adduct is usually obtained from about 0.5 to about 10alkylene oxide units per imidazole. The propylene oxide adduct ofimidazole is available from Dixie Chemical Company under the tradedesignation AP-5. The hydroxyl number (as determined by the method inASTM volume 08.02, p 608, method D2849.30.1.3 (1984) is about 406.5 mgKOH/g sample or about 138.0 g/eq. AP-5 is a viscous liquid having theempirical formula of C₆ H₁₀ N₂ O (MW 126) according to the manufacturerand contains about one propylene oxide per imidazole.

The alkylene oxide adduct of imidazole is employed in amounts of about0.1% to about 10% by weight and preferably about 1% to about 5% byweight based upon the total weight of the cycloaliphatic epoxide,anhydride hardening agent, and alkylene oxide adduct of imidazole. Thealkylene oxide adduct of imidazole promotes the hardening of thecycloaliphatic epoxide and also enters into reaction and, thereby, isnot free to migrate and does not promote corrosion in the curedencapsulated product.

The compositions of the present invention can also include fillers suchas silicon dioxide (SiO₂), aluminum oxide (Al₂ O₃), tantalum pentoxide(Ta₂ O₅), silicon carbide (SiC), boron carbide (B₄ C), tungsten carbide(WC), silicon nitride (Si₃ N₄), and lithium aluminum silicate compounds.Selection of the proper filler, filler mixture ratios, and fillerloading is based on the potential of the fillers to lower thecoefficient of thermal expansion (CTE) of the resulting composite andcan be determined by persons skilled once aware of the presentdisclosure without undue experimentation. The unfilled cycloaliphaticepoxy has a CTE (coefficient of thermal expansion) of 95 ppm/° C. As thefiller concentration is raised, a linear decrease in CTE following thelinear rule of mixtures is obtained. This is observed for all thefillers listed with Al₂ O₃ exhibiting the largest reduction in CTE (45%)and tantalum pentoxide with the least improvement (20%) at a fillerloading of 95% by weight. In these examples, the fillers were nottreated with any interfacial adhesion promoters.

If fillers are employed in the composition, these are usually present inamounts of about 10% to 98% by weight, and preferably about 90% to 95%by weight.

The compositions of the present invention exhibit a viscosity of about50 centipoise to about 5,000 centipoise at about 25° C. and more usuallyabout 100 centipoise to about 1,000 centipoise at about 25° C., typicalof which is about 700 centipoise at about 25° C.

The compositions are generally prepared by rapidly admixing immediatelyprior to introduction in the mold, the epoxy employed with a mixture ofthe anhydride and the promoter under pressure conditions of about 500psi to about 3,000 psi and typically about 1,000 psi. In the eventfillers are employed, the pressure is usually about 1,500 psi to about2,000 psi. If a filler and/or flexibilizer is employed, such can beprovided with either the epoxy or the mixture of anhydride and promoteror partially with both.

The composition is then introduced into the mold for the encapsulation.The pressure in the mold is about 25 psi to about 50 psi which isadvantageous since the electronic part being encapsulated will not beexposed to the type of high pressure employed in transfer moldingoperations. The temperature of the mold is usually about: 100° C. toabout 200° C. and more usually about 130° C. to about 160° C. The timein the mold is from about 30 seconds to about 10 minutes and morepreferably about 1 minute to about 5 minutes, shorter times beingemployed for use at higher temperatures. The encapsulated article isremoved from the mold when it is in a tack-free state. It can then besubjected to a post-cure such as heating at about 150° C. to about 200°C., typical of which is about 185° C. for about 1 hour to about 3 hours,preferably about 2 hours.

Example of typical microelectronic devices that can employ thecomposition of the present invention can be found in Surface MountTechnology, G. Caswell, Chapter 1, International Society for HybridMicroelectronics, 1984, disclosure of which is incorporated herein byreference.

The following non-limiting examples are presented to further illustratethe present invention.

EXAMPLE 1

A composition containing about 6.0 parts by weight of3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexanecarboxylate (Union CarbideERL-4221) and about 4.0 parts by weight of a Union Carbide ERL-4350 (aproprietary cycloaliphatic flexibilizing epoxy of Union Carbide); above8.0 parts by weight of methylcyclohexane-1,2-dicarboxylic anhydride(about 89% of the epoxide equivalent of the cycloaliphatic epoxide) andabout 0.4 parts by weight of propylene oxide adduct of imidazole (DixieChemical Company-AP-5).

The composition is subjected to a temperature of about 150° C. andexhibits a gel time thereat of less than 2 minutes. The gel time is theamount of time to provide a tack-free composition that can be removedfrom the mold.

EXAMPLE 2

A composition containing about 5.2 parts by weight of3,4-epoxycyclohexylmethyl 3,4-cyclohexylcarboxylate, about 10.7 parts byweight of a polypropylene glycol (Union Carbide Niax LHT-34), about 5.4parts by weight of methylcyclohexane-1,2-dicarboxylic anhydride, andabout: 0.25 parts by weight of propylene glycol adduct of imidazole(Dixie Chemical Company AP-5). The composition is subjected to atemperature of 100° C. and exhibits a gel time of about 21 minutes.

EXAMPLE 3

A composition containing about 7.0 parts by weight of ERL-4221, about3.0 parts by weight of ERL-4350, about 8.7 parts by weight ofmethylcyclohexane-1,2-dicarboxylic anhydride, and about 0.19 parts byweight of AP-5. The composition is subjected to a temperature of 100° C.and exhibits a gel time of about 23 minutes.

Example 3 is repeated, except that about 0.37 parts by weight of AP-5are used. The gel time at 100° C. is about 15 minutes.

EXAMPLE 5

Example 3 is repeated, except that about 0.56 parts by weight of AP-5 isused. The gel time at 100° C. is about 8 minutes.

Comparison Example A

ERL-4221 (about 10 parts by weight), NIAX LHT-34 (about 20 parts byweight), and methylcyclohexane-1,2-dicarboxylic anhydride (about 10parts by weight) were mixed with various levels of pyridine andsubjected to 100° C. The respective gel times are shown below.

    ______________________________________                  Parts by                          Gel Time (min)    Promoter      Weight  at 100° C.    ______________________________________    None          --      >221    Pyridine      0.12    60    Pyridine      0.24    57    Pyridine      0.50    56    ______________________________________

Comparison Example B

ERL-4221 (about 10.0 parts by weight), NIAX LHT-34 (about 15.5 parts byweight), and methylcyclohexane-1,2-dicarboxylic anhydride (about 10.4parts by weight) were mixed with various promoters and subjected to 100°C. The respective gel times are reported below.

    ______________________________________                      Parts by                              Gel Time (min)    Promoter          Weight  at 100° C.    ______________________________________    Pyridine          0.12     52    2,3-Diazabicyclooctane                      0.12     45    2-Ethyl-4-Methylimidazole                      0.24    >60    Benzyldimethylamine                      0.24    >72    Morpholine        1.2     >126    ______________________________________

Comparison Example C

ERL-4221 (about 7.0 parts by weight), ERL-4350 (about 3.0 parts byweight), and methylcyclohexane-1,2-dicarboxylic anhydride (about 8.7parts by weight) were mixed with various promoters and subjected to 100°C. The respective gel times are reported below.

    ______________________________________                      Parts by                              Gel Time (min)    Promoter          Weight  at 100° C.    ______________________________________    Pyridine          0.19    20    2-Ethyl-4-Methylimidazole                      0.19    >78    Benzyldimethylamine                      0.19    57    ______________________________________

The 100° C. used in Examples 2-5 and Comparison Examples A-C providesfor a very convenient screening or testing temperature with the actualtemperature more usually employed in the RIM process being about 150° C.Those compositions exhibiting gel times of about 25 minutes or less at100° C. normally exhibit gel times of less than 2 minutes at 140° C. to160° C. As apparent from a comparison of Comparison Examples A-C withExamples 1-5, the use of the alkylene oxide of the imidazole providesfor significantly improved reaction times as compared to the variouspromoters tested, except for certain of the examples with pyridine.However, pyridine is not especially suitable since it is a monomericmaterial that remains as such in the cured polymer as a potentiallytroublesome free amine.

Although the invention was described for an application of workpieceencapsulation, and with the cure time of the reactants being speeded upby the use of localized heat upon dispensing, the intent and features ofthe invention are useful and intended to be used for other types ofapplications. It will be apparent to those skilled in the art thatmodifications to the disclosed embodiments can be effected withoutdeparting from the spirit and scope of the present invention.

What is claimed is:
 1. A process for dispensing onto a plurality ofworkpieces a combination substance formed from a plurality of materials,said combination substance being a mixture having at least twocomponents which react at a first rate with each other at a firsttemperature to convert said combination substance into a gelled materialand which react with each other at a second rate at a secondtemperature, said second temperature being lower than said firsttemperature and said second rate being slower than said first rate,thereby keeping said combination substance ungelled at said secondtemperature for a longer time than at said first temperature, comprisingthe steps of:mixing a quantity of said combination substance at saidsecond temperature in a mixing chamber; dispensing a portion of saidmixed combination substance onto each one of said plurality ofworkpieces from an output port of a dispense head connected to saidmixing chamber; and selectively raising the temperature only of eachsaid portion of said combination substance being dispensed to said firsttemperature with a heater located at the output port just before saideach portion is dispensed onto one of said workpieces.
 2. A process asin claim 1 further comprising the step of preheating each of saidplurality of workpieces prior to said dispensing of said combinationsubstance.
 3. A process as in claim 1 wherein said combination substanceencapsulates each of said plurality of workpieces and said combinationsubstance cures to a permanent state.
 4. A process as in claim 1 whereinsaid plurality of workpieces are mounted on a tape reel means.
 5. Aprocess as in claim 1 wherein said first temperature is sufficientlyhigh to convert said controlled portion into a gelled material within 10minutes after being dispensed onto one of said workpieces.